This invention relates to a communication system for a point-to-multipoint communication, in particular, to a communication system that each of communication terminals transmits a data frame(s) thereof to a server during an assigned frame transmission period assigned thereto by the server.
Recently, with the spread of multimedia communication service for supplying sound, pictures and/or connection to internet, a demand for increasing transmission speed and reducing a production cost of a network becomes higher. There is such a demand regarding not only a backbone network but also a subscriber access network.
To meet the demand regarding the subscriber access network, Ethernet technology is applied to a subscriber access network of a public network. Ethernet technology has been developed for a local area network.
Attention is given to an application of the Ethernet technology to a passive optical network (hereinafter abbreviated as PON) that is a typical point-to-multipoint optical communication system of a shared transmission media type. Hereinafter, the PON applied with the Ethernet technology will be referred as an Ethernet-PON (or EPON) system.
A related EPON system comprises a plurality of subscriber's units (or terminals) and a station's unit (or server). The subscriber's units separately are connected to first optical fibers while the station's unit is connected to a second optical fiber. The first optical fibers are coupled to the second optical fiber in common through an optical coupler (or a passive optical device). Thus, the subscriber's units are connected to the second optical fiber in common.
The station's unit repeatedly assigns frame transmission periods to the subscriber's units in turn to avoid a collision of data frames from two or more subscriber's units. Each of the subscriber's units transmits data as the data frame(s) to the station's unit during the assigned frame transmission period assigned thereto by the station's unit.
If a certain subscribers unit has no data which should be transmitted to the station's unit, the assigned frame transmission period assigned thereto is wasted. This deteriorates communication efficiency of the EPON system. Thus, it is desirable that the time length of the frame transmission period is adjustable according to volume of the data held by each subscriber's unit.
However, it is difficult to detect (change of) the volume of the data held by each subscriber's unit for the station's unit. Accordingly, the EPON system must have a complicated structure to detect the volume of the data held by each subscriber's unit. This increases the production cost of the EPON system.
Furthermore, even if the station's unit can detect the volume of the data held by each subscriber's unit, there is a case where the station's unit assigns the frame transmission period longer than a necessary time for transmitting the data. For example, the station's unit assigns the frame transmission period having predetermined time length to the subscribers unit having no data. Thus, if the subscriber's unit does not receive new data which should be transmitted to the station's unit, the assigned frame transmission period is wasted.
As mentioned above, the related EPON has a problem of bad communication efficiency.